The invention relates to a method for spraying and to a system for spraying a coating liquid.
Spray systems comprising a rotary atomizing element in the form of a so-called bell to atomize and spray coating liquids onto an object to be coated are known from the U.S. Pat. Nos. 4,275,383 and 4,505,430; German patent documents 30 00 002 A1 and 35 09 874 A1. They disclose applying a high electrical potential, which may be positive or negative, to the rotary atomizers and/or to the spray coating liquid. Typically the high voltage is in the range of 4 kv to 140 kv. A high-voltage spray system fitted with an irrotational spray nozzle is known from U.S. Pat. No. 3,731,145.
Some spray particles of the sprayed coating material tend to migrate rearward from the spray jet and to precipitate on the front terminal outer surface zone of the spray system where they will cure and constitute a hard coating.
The objective of the invention is to prevent or at least reduce adhesion and curing/hardening and the ensuing coating, i.e. layering of the coating material on the front terminal outer surface zone of the spray system.
Accordingly the invention concerns a method for spraying coating liquids wherein such a coating liquid is sprayed from a liquid atomizer in the form of an irrotational nozzle or preferably in the form of a rotating, rotary atomizing element onto an object to be coated, and said method is characterized in that the terminal zone of an outer surface of the spray system delivering the spray jet of coating liquid is wetted with an accessory liquid in order to prevent or reduce layering of coating liquid precipitated on said surface.
The invention also concerns a coating-liquid spray system containing a liquid atomizer in the form of an irrotational nozzle or preferably in the form of a rotating rotary atomizing element to spray coating liquid onto an object to be coated, characterized in that it comprises a wetting unit to wet a front, terminal zone of the external surface of the spray system during spraying with an accessory liquid in order to prevent or delay drying and layering of coating liquid precipitated on said surface.
The invention offers the advantage that the accessory liquid prevents or strongly delays drying and layering of coating liquid on the spray system.
The wetting unit to wet the outer surface""s front terminal zone with said accessory liquid preferably contains one or more boreholes or nozzles through which the accessory liquidxe2x80x94which illustratively is in the form of liquid drops or of an aerosolxe2x80x94is deposited in metered manner on the pertinent system component, for instance on rotating liquid atomizers.
In a further feature, the invention contains a cooling unit to cool at least part of the spraying system. Because of lowered temperature, drying or layering of the coating liquid on the outer surface""s front terminal zone shall be prevented or much delayed. Accordingly cooling the outer surface""s terminal zone enhances the effectiveness of the accessory liquid, namely to prevent or reduce drying and layering of coating liquid on said surface.
Preferably a cooling unit to cool the coolant shall be integrated into the spraying system. Preferably a compressed gas, for instance compressed air, shall be used as the coolant and it is cooled by the cooling unit and blown onto the cooling portion of the spraying system.
The voltage applied to the spraying system may be higher than ground or another predetermined reference potential on part or all of said system. The higher voltage may be constant or variable and it may be positive or negative relative to the reference voltage.